Pulse slimming equalization techniques are effective and necessary in increasing the magnetic recording density of magnetic transitions in magnetic media by reducing intersymbol interference. Signal processing to slim the readback pulses to minimize interference among adjacent pulses in a pulse train derived from high density, high transfer rate regions of the media, increases readback reliability.
Investigations to improve magnetic recording or readback resolution have been reported for many years. Examples of such investigations appear in reports by; H. M. Sierra, "Increased Magnetic Recording Readback Resolution by Means of a Linear Passive Network", IBM J. RES. DEVELOP, Vol. 7, pages 22-23, January 1963; by G. B. Jacoby, "Signal Equalization In Digital Magnetic Recording", IEEE Transactions on Magnetics, Vol. MAG-4, No. 3, September 1963, and J. C. Vermdulen, "Read Pulse Compression by Linear Filtering", IBM Technical Disclosure Bulletin, Vol. 9, No. 10, pages 1272-1273, March 1967.
Richard C. Schneider, in a paper entitled, "An Improved Pulse Slimming Method for Magnetic Recording,", IEEE Transactions Magnetics Vol. MAG.-11, No. 5, pages 1240-1241, September 1975, describes an equalizer in a mass storage system comprising active circuit elements in two or more cascaded stages for slimming the pulse leading edge and the pulse trailing edge in sequence. When an integrator is used at the last pulse slimming stage to obtain zero crossings, it is noted that undesired negative side lobes or signal overshoot can be kept away from the base line.
T. Aikawa et al, in a paper entitled "An Experimental Study of Signal Equalization for Thin Film Heads", IEEE Transactions on Magnetics, Vol. MAG.-22, No. 5, pages 1209-1211, September 1986, describes two techniques of reducing negative side lobes in signal processing for thin film heads. One is an equalizer providing pulse slimming and elimination of negative side lobes. The equalizer is called a double cosign equalizer. Two consecutive delay lines are used to reduce the side lobes. The double cosign equalizer is said to increase signal amplitude and decrease the ripples. The other way of reducing ripples which is described is to provide a head with asymmetrical pole faces.
Robert Kost et al, discusses passive LC filters in a paper entitled "Arbitrary Equalization with Simple LC Structures", IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, pages 3346-3348, November 1981. The design of the output signal is presented in connection with a system using amplitude thresholding and signal peak detection. The design process for all-pole, LC ladder filters using two filter models is presented, comparing an insertion loss circuit realization with a multi-input circuit realization. The paper mentions a filter design, which is not illustrated, referred to as an equalizer to remove intersymbol interference in the time derivative of the slimmed output pulse. SPICE simulations of an input pulse and a slimmed output pulse, displaced in time, are presented.
Differential equalizers are not referenced in any of these papers. Aikawa et al would be forced to use matched delay lines for noise cancellation in pulse slimming. The use of a differential network, as provided in accordance with this invention is not treated in these papers. Delay line equalization, as proposed by Aikawa, is not readily adapted to a differential configuration. The delay lines are complex and are difficult to match.
Independent slimming of the pulse leading and trailing edges using consecutive active stages, as proposed by Schneider, is also complex and not easily or effectively adapted to a differential type of network configuration. Four interacting active circuit adjustments are required in the differential network.
The approach taken by Kost et al to achieve signal equalization using LC filters requires current inputs, FIG. 2b, which are difficult to adjust. One drive input and two feed forward, or gain controls, are shown. These also interact in the network. The gain controls require independent adjustments. The concept of injecting currents into nodes of an LC ladder network is not compatible with simplicity in signal slimming where multiple magnetic heads of differing gain in relation to one another and with respect to the magnetic media are employed.
These papers present a brief review of passive and active pulse equalization of the prior art presently known to the applicant, which is relevant to pulse slimming techniques in high density magnetic media storage systems.
The patents discussed below represent the results of a search for prior art with respect to this invention. In general, the relevance of these patents resides in applications of LC networks in processing signals derived from transducers in read back systems.
U.S. Pat. No. 4,208,634, Peek et al, describes a circuit for suppressing pulse shaped noise caused by scratches on a phonograph disk. This circuit is relevant only in the sense that an LC network is employed. Pulse slimming is not referred to.
U.S. Pat. No. 4,244,008, Holt, describes a read back circuit for a transducer scanning a floppy disk which employs a frequency equalizer comprising a differential LC filter coupled to the input of a differentiating amplifier. The thrust of this disclosure is in the use of a track counter to
control both circuits in the equalizer, to compensate peak shifting in readback.
U.S. Pat. No. 4,276,573, Halpern et al, describes a pulse shaping network for a transducer in a disk memory system which employs a capacity terminated, parallel cascade of LC filters, functioning as a sine pulse forming filter. The system is directed to reducing intersymbol cross talk, for the purpose of equalizing peak amplitudes and correcting peak phase shifts. Intersymbol cross talk is reduced using a network transfer function consisting of first and third half-period harmonics for optimizing pulse slimming.
U.S. Pat. No. 4,327,383, Holt, describes the use of an equalizing filter in a circuit having a transducer for reading digital data from a record medium. The equalizing filter has a response in a frequency domain expressed by a specific Laplace transform providing a substantially linear time delay and an acceptable gain rise in frequency ranges associated with tracks having higher packing densities, such as the middle to inner disk tracks. A track counter switches the outer track signals around the equalizing filter into the output which includes a low pass LC filter and a detector.
U.S. Pat. No. 4,344,093, Huber, describes a readback circuit including a transducer circuit for detecting data recorded on a magnetic medium and an optimum equalizer circuit receiving its input from the transducer circuit. The optimum equalizer is implemented by a passive LC, balanced ladder network, depicted as an eight pole filter. The non linear phase delay of the optimum equalizer is compensated by a second delay equalizer which is implemented by an all pass lattice network of capacitor inductor sections cascaded together. It is stated that the input read signal is transformed into a data representative signal which has improved signal to noise ratio performance and limited pulse widths.
U.S. Pat. No. 4,506,236, Cloke, describes an open-ended composite filter/differentiator which is used in the read/write channels of a disk drive system, for producing an even function and an odd function of a transfer function supplied thereto. A resistor is connected in series with an energy source and an LC network has its input coupled across the energy source and the resistor. Odd functions are derived across the resistor and even functions are derived across the capacitor terminated output of the LC network.
These patents describe various techniques for pulse shaping including LC ladder networks several of which, Huber and Halpern et al reference pulse slimming all, except Cloke, involve complex read/write channels. Some require switching, the two Holt patents, to control inputs or to change network frequency. Cloke requires zero crossing and threshold detectors together with qualification logic to process the odd/even transfer functions.
With continuing needs for the reduction in size coupled with increases in mass storage in such magnetic storage systems, higher recording densities and consequent pulse compression for writing and reading are required using circuits of simpler concept for pulse slimming and gain control.